1. Field of the Invention
The invention relates to a teletext, and in particular to a data decoding device capable of avoiding data error from incorrect sampling points caused by serious interference.
2. Description of the Related Art
Teletext is a popular service for European television broadcast, commonly providing information including TV schedules, current affairs and sports news, games and subtitling in different languages. Teletext comprises encoded data carried in the vertical blanking interval (VBI) of a television broadcast signal that temporarily suspends transmission of the signal, allowing scanning to return to the first line of the television screen to trace the next. Upon reception, a data slicer in a receiver compares the TV broadcast signal transmitted at the VBI with a slicing level to determine each bit representing the teletext data.
FIG. 1 illustrates a waveform and corresponding slicing result of a TV broadcast signal. The VBI can be divided into clock-run-in, start code (not shown in FIG. 1), and teletext data. The slicing level is typically determined by the signal amplitude during the clock-run-in interval, and is set to assess each bit carried by teletext to be either 0 or 1. The slicing level, for example, can be the average amplitude of the TV broadcast signal received during clock-run-in interval. However, the TV broadcast signal experiences interference including environmental noise and group delay during data transmission, such that signal quality of the TV signal degrades, leading to false data determination.